I don't use that kind of dye or watercolours (I might give it a go though)
My method is to take some of my own hair dye (I use manic panic/special effects/SCREAM)
And select a pink colour (Flamingo or something along those lines, I get a bowl and I add one part dye to 2-3 parts shampoo (Depending on the darkness of the colour) and I mix it up and lather it in my pony's hair until it goes bubbly and rinse it off, it needs a few times doing sometimes but this way you can build it up and make sure you don't over pink.
I've also done this using conditioner and soaking..
I'll have to take some pictures one day